Mordant



Patented May 7, 1935 UNITED STATES PATENT OFFICE MORDANT No Drawing. Application March 1, 1934, Serial No. 713,532

7 Claims.

This invention relates to an improved mordant for use of engravers, and is particularly directed to nitric acid which contains a small amount of ammonium sulfate.

In photo engraving, line engraving, etc. a metal plate, usually zinc or copper, is etched with-a mordant. The technique is, of course, well known and no extended discussion seems necessary.

In order to obtain the best results, the mordant must act quickly and evenly on the metal, and must act slowly, if at all, on the groundand resin usually used to block the action of the mordant on selected areas. The mordant should not discolor the work, and, from the standpoint of economy, should have as long a life as possible.

Nitric acid has found extensive use as a mordant, but its efficiency is rather poor, its action is not always uniform, and it sometimes discolors the work.

It has been proposed to add sodium sulfate, alum, or sulfuric acid to nitric acid, but while some improvement is noted, the resulting mixtures are none too satisfactory.

I have found that the addition of ammonium sulfate to nitric acid appreciably enhances its value as a mordant. Ammonium sulfate is somewhat similar, chemically, to the agents heretofore mentioned, but the nitric acid containing it exhibits many excellent properties and constitutes an eminently satisfactory mordant, especially for zinc. The period of effectiveness of the nitric acid is increased. Moreover, my improved mordant acts evenly on the exposed zinc and imparts to etched surfaces a characteristic white color.

My improved mordant is preferably made by adding the desired amount of ammonium sulfate to a relatively concentrated acid. The mixture is diluted with water prior to its employment as an etching bath. Of course, the ammonium sulfate can be added to dilute acid if desired.

The following example illustrates one embodiment of my invention:

6.75% (by weight) of ammonium sulfate was added to a 40 B. nitric acid. This constitutes a product of my invention. The product, for test purposes, was diluted to make a 10% HNO3 bath, and a plate of photo-engraving zinc, coated on sides and back, was subjected to its action in a splash tub of conventional design. The etching action was quite even, and the zinc surface was clean and white. This improved bath showed an increasedperiod of effectiveness.

While my specific example shows the use of 6.75% ammonium sulfate, based on 40 B. nitric acid, I do not intend to be limited thereby. I may use somewhat larger amounts of ammonium sulfate, though above about 10% the mixture exhibits an uneven etching action. I may use smaller amounts than 6.75% ammonium sulfate, since below about 10% the effect of the ammonium sulfate seems to be a function of the amount. Minute amounts have some effect, and from a commercial standpoint it may sometimes be desirable to use very small quantities of ammonium sulfate to obtain, at low cost, some of its beneficial effect.

Generally, I prefer to use no more than about 10% ammonium sulfate, based on 40 B. nitric acid, while more specifically I prefer to use no more than about 6.75%.

I claim:

1. A mordant for engraving comprising nitric acid and a minor amount of ammonium sulfate.

2. A mordant for engraving comprising nitric acid and no more than about ten per cent by weight of ammonium sulfate, based on 40 B. nitric acid.

3. A mordant for engraving comprising nitric acid and no more. than about six and seventy five hundredths per cent by weight of ammonium sulfate, based on 40 B. nitric acid.

4. In a process of engraving the step comprising etching metal with a mordant comprising nitric acid, and a minor amount of ammonium sulfate.

5. In a process of engraving the step comprising etching metal with a mordant comprising nitric acid and no more than about six and seventy five hundredths per cent by weight of ammonium sulfate, based on 40 B. nitric acid.

6. In a process of engraving the step compris ing etching metal with a mordant comprising nitric acid and no more than about ten per cent by weight of ammonium sulfate.

7. In a process of engraving the step comprising etching zinc with a. mordant comprising nitric acid and no more than about ten per cent by weight of ammonium sulfate.

ERNEST R. BOLLER. 

